"The fact is that Christmas, as we know it, is a rather modern innovation. Christ's birthday was not celebrated until more than 300 years had gone by, years in which accurate birth records (if there were any) had been lost. The early church remembered and celebrated Christ's Resurrection from the dead, which was more important. But the church was slow in adding Christmas to its list of dates worthy of recognition.
Luke pinpoints the era of Christ's birth by naming Augustus as Rome's imperial ruler. Roman history shows that Caesar Augustus was born 691 years after founding of the City of Rome. Luke 2 further tells that Cyrenius was Syria's governor; again, thanks to Rome's exhaustive record of names and events, historians have determined what is believed to be the particular census that Luke described. These dates have minor discrepancies; yet secular history gives us almost the exact year of Christ's birth.
But the month? the day? Winter was wet and chilly.in Judea. It is unlikely that shepherds would have spent a December night in an open field, subject to rain and wind. Christ's birth was more likely during the spring lambing season, when nights would have been balmy and shepherds would have needed to be awake, tending the ewes.
A pagan festival, Natalis Invicti, was a Roman affair celebrated on December 25, when the sun was in its winter solstice. Worshipers of the Roman sun god enthusiastically pulled their Christian friends into the partying. By A.D. 386, church leaders set up the celebration of "Christ Mass" ("Christ's Coming"), so that Christians could join the festival activities without bending to paganism.
After the Roman Empire dissolved, Christians continued the December 25 birthday custom. By that time, December 25 seemed more fitting than any other date."